iPhone 6s Plus activated

I found a very good offer for iPhone 6s Plus, the seller says it's activated for a client who never picked it up, my question, can the client claim it or block it? In other words, is it possible that I face problems with my access to the phone becasue of this activation?

If it were me, I would ask the seller to clarify his statement of "it's activated for a client...". Can that activation be cancelled or transferred over to you since the client never picked it up? Did the client get a refund? See my point? If the iCloud activation lock has been enabled on the device, you will not be able to use the device as your own.